In competitive go-kart racing, the kart designer strives to transmit as much of the motor's propulsive power to the drive axle as possible by minimizing power losses between the engine's output drive shaft and the drive axle which turns the wheels. To minimize power loss, the designer seeks to eliminate or at least minimize friction, vibration, structural deflection, and the like in all the components between the output drive shaft of the motor and the drive axle. On racing go-karts, the primary component interacting with the output drive shaft is a centrifugal clutch drive system.
Centrifugal clutch drive systems are well known in the art. These clutches, sometimes known as frictional contact axial clutches, utilize mating frictional members to transfer torque from an input shaft (the motor's output drive shaft) to an output sprocket or gear. A chain or belt transmits the torque from the output sprocket or pulley to the drive axle which turns the wheels of the go-kart. In one configuration, torque is transferred by harnessing the effects of centrifugal force upon pivoted weights to generate axial movement and ultimately axial thrust. This axial thrust is applied upon an output frictional member which, by interacting with an input shaft frictional member, effectively transmits the input shaft torque to the output sprocket or pulley.
A typical centrifugal clutch drive system includes a clutch mechanism attached to the motor's output drive shaft. An output sprocket is attached to a housing which is rotatably mounted about the drive shaft of the motor so as to rotate independently of the drive shaft. When the drive shaft reaches a specific RPM, the clutch mechanism engages the housing, thereby spinning the sprocket to drive a chain or belt. Anti-friction elements, such as needle roller bearings or bushings, are used to rotatably mount the sprocket about the drive shaft. By virtue of their design, needle roller bearings and bushings can accommodate small sprockets (numerically fewer teeth). Needle roller bearings and bushings, however, can resist only radial loads, not axial loads because they are free to float fore and aft in the axle direction.
Vibration and bending of the input shaft due to the asymmetric load of the chain can cause significant lateral displacement of the sprocket. The chain determines the axial position of the sprocket. Chains, however, do not have much lateral strength or stiffness. Consequently, they quickly wear out or stretch and waste energy if not properly aligned. The fore and aft axial displacement of the sprocket also causes accelerated wear between the drum and the discs in disc clutches or drum and shoes in rim clutches.
Needle roller bearings and bushings are relatively imprecise and allow the sprocket to wobble, thereby wasting the propulsive energy of the motor. Adding to the imprecision, the clutch manufacturer, not the bearing manufacturer, often machines the inner bearing race so as to size the race to fit upon the drive shaft of the motor. The clutch manufacturer, however, cannot machine the bearing race with as tight of tolerances as the bearing manufacturer. Thus, the machining of the inner race by the clutch manufacturer introduces unwanted play between the bearings and bushings and the drive shaft of the motor, leading the sprocket to wobble, causing energy loss.
Needle roller bearings are considered “open” in that they have no seals to prevent lubricant from flowing out and dirt from blowing in. The loss of lubricant and the influx of dirt cause rapid wearing of the needle roller bearings and increased friction, which wastes the propulsive energy of the motor. Light thrust washers or flanged bushings are sometimes used with needle roller bearings, but are typically applied in only one axial direction due to space limitations. Lubrication of these thrust washers is minimal or nonexistent. While thrust washers are intended to inhibit sprocket wobble, they are largely ineffective.
Unlike needle roller bearings and bushings, ball bearings can withstand both axial and radial loads. Some centrifugal clutches use ball bearings, but these are typically positioned between the drum and drive hub. In that position, the ball bearings are not easily accessible for maintenance or replacement. Often the ball bearing assembly is press fit into place making removal difficult and time consuming. Also, because these centrifugal clutches position the inner race of the ball bearings around the drive hub, the ball bearings are larger and heavier than necessary. Consequently, the large ball bearings limit the size (number of teeth) of the sprocket that can be used. In other words, with the large ball bearings, smaller sprockets (low number of teeth) cannot be used.
In some centrifugal clutches, the centerline of the bearing, whether it be a needle roller bearing or a ball bearing, is not aligned with the centerline of the chain engaging the sprocket. This misalignment causes an overturning moment on the bearing, thereby increasing the rolling resistance of the bearing. This wastes the propulsive energy of the motor.
What is needed, therefore, is a centrifugal clutch drive system that minimizes power losses between the motor and the drive axle.